Examples of such combinations are television sets and computer monitors. Typically, the monitor comprises a cathode-ray-tube (CRT). A problem that may occur if loudspeakers are integrated with such a monitor relates to the acoustic coupling between the loudspeaker and the monitor's shadow mask. To explain this in a more detail, the configuration of a CRT and the working of a bass-loudspeaker are discussed below.
A CRT includes an electron gun, deflection coils and a screen. The inside of the screen is coated with light-sensitive phosphors. In color CRTs; the phosphors comprise red, blue and green crystals arranged in triads of each one of the colored phosphors. The electron gun generates three beams, one for each color, which directed by the deflection coils scan the screen moving from left to right and up and down. The phosphors light up in the relevant color when illuminated by the beams. In order to produce a correctly colored image, each electron beam must hit only the appropriate colored phosphor. To ensure such accuracy, the shadow mask is used as a filter. The shadow mask is a metal plate of the same shape as the screen and with the same number of holes as the number of triads on the screen. Each hole has a corresponding triad of phosphors which prevents adjacent triads from being influenced.
A loudspeaker is an electro-acoustic transducer, i.e., a device for converting electrical energy into acoustic energy by moving a diaphragm under control of an electric signal. The moving diaphragm displaces a volume of air, the resulting pressure variations being perceived as sound. As known in the art, loudspeakers are mounted in an enclosure or speaker cabinet.
The CRT's mask turns out to be susceptible to frequencies in the audio spectrum, i.e., it will resonate at particular frequencies. This mechanical vibration adversely affects the quality of the picture being displayed due to the resulting misalignment between the mask's holes and the corresponding triads on the screen. Sound waves may be incident on the CRT through the air or via mechanical contact, directly or indirectly, between the CRT and the loudspeaker. The problem is mostly noticeable in the low and middle frequency ranges.
Several solutions are known to the well known problem of vibrating masks. For example, U.S. Pat. No. 4,318,025 suggests to suspend the mask from the screen through an absorbing material. U.S. Pat. No. 5,103,132 and U.S. Pat. No. 5,315,208 propose using a specific supporting frame to reduce the mask's sensitivity to vibrations (Philips prior art).
Although the problems are mostly noticeable in CRTs, other types of monitors may also be affected by undesired vibrations of their components caused by acoustic interaction with a nearby loudspeaker system.